O'Brien currently has four of the main contenders in the Derby betting - St Nicholas Abbey, Jan Vermeer, Midas Touch and Cape Blanco.

"Johnny Murtagh rode St Nicholas Abbey this morning and thought he might not have shown the same sparkle he showed last Tuesday morning, therefore he won't make up his mind what horse he rides in the Derby until the middle of next week," O'Brien told the Coolmore website.

St Nicholas Abbey was undefeated as last season's champion two-year-old but could finish only sixth behind Makfi in the 2000 Guineas at Newmarket while the other three have all won their Derby trials.

Speculation over Murtagh's ride has been increasing all week, and after the latest statement, Jan Vermeer has been backed down to about 3-1 favouritism (from 6s) while St Nicholas Abbey is 7-2 from 2-1.

At no stage, has the Ballydoyle-based Coolmore team, for which O'Brien trains, suggested there is a specific problem with St Nicholas Abbey.

An interesting sub-plot is that former retained O'Brien rider Kieren Fallon appears to be waiting to see if he can pick up one of the spare Ballydoyle mounts.

Trainer Mark Tompkins had been keen to book Fallon for his contender Ted Spread after jockey Darryll Holland broke his collarbone in a fall.

But Tompkins said he could not get a commitment from Fallon and has booked Michael Hills instead.

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